1. Field of the Invention
This invention relates to a focus detecting apparatus in a camera or the like using, for example, a TTL phase difference detecting system, and particularly to a focus detecting apparatus for detecting the in-focus state in a plurality of areas in the image plane of a photo-taking lens.
2. Related Background Art
Generally, a focus detecting apparatus as shown in FIG. 1 of the accompanying drawings is known as a focus detecting apparatus used, for example, in a camera or the like. In FIG. 1, the reference numeral 1 designates a photo-taking lens, the reference numeral 2 denotes a field mask, the reference numeral 3 designates a condenser lens, the reference numeral 4 denotes a stop mask, the reference numeral 5 designates a re-imaging lens, and the reference numeral 6 denotes a photoelectric conversion element array. The reference numerals 7 and 8 designate the exit pupil and the optical axis, respectively, of the photo-taking lens 1.
In the thus constructed focus detecting apparatus, a standard portion light beam and a reference portion light beam transmitted through the photo-taking lens 1 are photoelectrically converted by the photoelectric conversion element array 6 and the focus is detected.
A focus detecting apparatus of this kind is disclosed in Japanese Laid-Open Patent Application No. 63-11906.
This apparatus comprises a first photoelectric conversion element array and a second photoelectric conversion element array disposed at a position containing the optical axis of a photo-taking lens and a position not containing the optical axis, respectively, the direction of said second photoelectric conversion element array being set to a direction substantially perpendicular to a straight line passing through the second photoelectric conversion element array and the optic axis.
However, in the focus detecting apparatus of this kind, of focus detection areas which do not contain the optical axis of the photo-taking lens, any focus detection area does not intersect another focus detection area which does not contain the optic axis, in the direction of detection, and focus detection has been impossible for any object which does not have a luminance distribution in a direction of detection fit for a focus detecting optical system.
There is also a focus detecting apparatus according to the prior art which, as disclosed in Japanese Laid-Open Patent Application No. 62-95511, makes focus detection possible even for any object which does not have a luminance distribution in a direction fit for a focus detecting optical system.
This apparatus comprises a secondary imaging system and a plurality of pairs of photoelectric conversion means so that the image on the predetermined focal plane of a photo-taking lens may be separated into two secondary images and re-imaged by a secondary imaging system and the directions of change in the relative spacing between these two images may differ from each other.
In the focus detecting apparatus according to the prior art, however, when the direction of detection of a focus detection area is perpendicular to a straight line passing through this focus detection area and the optical axis, a pair of light beams forming the secondary images have been eclipsed in a direction nearly perpendicular to the change in the relative position and on the other hand, when the optic axis lies on the extension of the direction of detection of the focus detection area, the pair of light beams forming the secondary images have been eclipsed in the direction of change of the relative position. As a result, only the optical system in a particular direction is greatly affected by the eclipse and focus detection becomes impossible, and this has led to the problem that the reliability of focus detection is reduced.